1. Field of the Invention
The present invention relates to alloys of gold color and, more particularly, to alloys that simulate gold in spectral appearance, tarnish resistance and mechanical properties, and that are used in such products as coinage, giftware, kitchenware, and other elegant metal objects.
2. The Prior Art
From very earliest times, gold has been a metal of special interest because of its extraordinary spectral, chemical and mechanical characteristics, i.e. its specular reflectance, tarnish resistance and ductile behavior.
Although ancient royalty often employed gold-based tableware and vessels, for the past four or five centuries the more usual high quality implements have been based on sterling silver. Typically this metal consists of about 92.5% silver and balance of copper, which eliminates gassiness that occurs when pure silver solidifies. Inexpensive flatware, dishes, bowls, etc., have been based on the use of nickel silver, a family of Cu-Ni-Zn alloys, which after finishing have been plated with pure silver. The base alloy in this case has a yellowish color, which although whiter than brass, is noticeable immediately when the plating is worn away.
One reason why silver has never become a competitor of gold in some fields is that tarnishing of silver alloy or plated silver articles results from contact with sulfurous atmosphere and is objectionable because of the hand-polishing usually required to maintain brightness. Metallurgists and artisans have tried for centuries to improve the tarnish resistance of such silver articles by judicious alloying, but without success. Efforts to reduce annoying tarnishing in such articles also have involved depositing, on silver surfaces, other metals having greater nobility than silver, including gold, platinum, palladium and rhodium, all of which are unduly expensive when so used.
Also low cost flatware has been fabricated from stainless steel. But, although stainless steel has good tarnish resistance, its appearance is that of a base metal.
The appearance of untarnished gold remains a very desirable objective for the fabrication of low cost jewelry, tableware, giftware, etc.
Copper alloys are of particular interest in the simulation of gold because of the inherent reddish color of elemental copper. Copper alloys have included: brasses, which generally differentiate from gold because of their bright yellow appearance; and bronzes which generally differentiate from gold because of their dull brown appearance. Furthermore, attempts to modify the optical properties of these brasses and bronzes often have been accompanied by unacceptable changes in their tarnish resistance.
A variety of copper alloys have been studied for their general interest, as well as for their relevance to gold simulation. A general study of one such alloy, P. H. Stirling, B.Sc., Ph.D., A.R.I.C., Junior Member., and Professor G. V. Raynor, M.A. D.Sc., Vice President (both of the University of Birmingham), entitled, "The Copper-Rich Alloys Of The System Copper-Aluminum-Indium," was reported in the Journal Of The Institute Of Metals, 1955-56, Vol. 84. This article discussed the metallurgy of Cu-Al-In alloys in detail, but did not address any specific metallurgical relationships that were intended to provide a marked similarity to gold in specular reflectance, tarnish resistance and ductile behavior.